Each edge of a cube is expanding at a rate of 4cm/s.
a) How fast is the volume changing when the edge is 5cm?
b) at what rate is the surface area changing when the edge is 7 cm?
If $\displaystyle s$ is the length of one side of a cube, then its volume is $\displaystyle V=s^3$. We are looking for $\displaystyle \frac{\,dV}{\,dt}$, so that means that $\displaystyle V=s^3\implies\frac{\,dV}{\,dt}=3s^2\frac{\,ds}{\,d t}$
You know the rate at which $\displaystyle s$ is changing, and you know the length of the side at that instant in time. Do you think you can complete the problem now?